List of abbreviations
of micros-
specialist terms
explained in
English +

Every attempt was made to provide correct information and labelling, however any liability for eventual errors or incompleteness is rejected!

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Dr. med.
H. Jastrow

of use
Overview veins (Venae):
(labelling in preparation)
wall of a vein: endothelium + intima
and a long smooth muscle cell (monkey)
Intima of a human
vein at the ankle
idem other area
detail thereof: endothelial cells
showing sectioned nuclei (human)
smooth muscle cell in the
subendothelial layer (human)
detail: smooth muscle cell  + endothelial cells
with widened intercellular spaces (human)
endothelial cell with widened 
intercellular spaces (human)
detail thereof:
subendothelial layer 2
detail thereof: cytoplasm + organells
of the endothelial cell (human)
Veins (Terminologia histologica: Venae) are large blood vessels carrying blood to the heart. With the exception of the pulmonal veins which bring oxygenated blood to the left atrium of the heart all other veins transport blood which is poor in oxygen. The walls of veins resemble those of arteries, however since the blood pressure is much lower in veins the different layers are thinner and often less clearly bordered against each other.
Looking from the interior outwards the wall of a vein has the following layers:
1. Tunica intima a squamous flat endothelium borders the lumen. A subendothelial layer (Stratum subendotheliale) is located beyond its basal lamina which is bordered by an internal elastic membrane (Membrana elastica interna).
The Stratum subendotheliale (= Lamina propria intimae) has fine reticularly ordered elastic fibres, a lot of collagenous fibres and shows singly lying intermingled smooth muscle cells.
In comparison to arteries the Membrana elastica interna is quite poorly formed, often it is non-continuous, and even may lack completely. The membrane consists of interconnected elastic fibres and marks the border to the media.
2. Tunica media consists of larger lamellas and interconnected membranes of elastic fibres and collagenous fibres many as well as branched smooth muscle cells. In comparison to arteries of equal diameter the media of veins is considerably thinner and the amount of smooth muscle cells, which are mainly in circular orientation, is reduced. The thickness of this muscular layer is larger in veins which have to resist orthostatic pressure (veins of the legs) and it is less in veins belonging to the gut.
3. Tunica externa (Adventitia) This layer is strongest in veins of the abdominal cavity and otherwise also remarkable. In contrast to arteries, there is no external elastic membrane (Membrana elastica externa) on the border to the media isolated fasciculi of elastic fibres are present instead. In larger veins of the arm or legs or in the internal jugulary vein elastic fibres form large compact networks. The mainly loose connective tissue has some amorphous fundamental substance several larger and many smaller collagenous fibres running in different directions. Further, fibrocytes and free connective tissue cells are located here. The adventitia serves for anchoring the veins to surrounding tissues and contains small vessels for blood supply of the wall of the vein itself (Vasa vasorum) as well as few non-myelinated nerve fibres. Additional bundles of interconnected strands of longitudinally oriented smooth muscle cells are encountered in the adventitia of abdominal veins (V. cava inferior, V. portae, V. iliaca, V. renalis, V. lienalis) and V. azygos.
Most veins are of intermediate size, i.e. have diameters of 2 - 9 mm. Such veins partly have a stronger layer of elastic fibres (Membrana elastica interna) with in some cases additional longitudinally oriented strands of smooth muscle cells (Vv. popliteae, femorales, cephalicae, mesenteriae, uterinae). The circular to spirally oriented muscle fibre bundles of the media are rich in cells in the veins of the leg whereas smooth muscle cells are poor in veins of more upper regions of the body. However, the adventitia is thick in most cases. There is considerable variation in relative amount of elastic fibres, collagenous fibres and intermingled smooth muscle cells.
Large veins with diameters of > 1 cm usually show a thicker subendothelial layer of the intima with only few smooth muscle cells. Their adventitia is quite thick and in some cases shows additional longitudinally oriented bundles of smooth muscle cells. In case of the upper caval vein (V. cava superior) a further outer layer of heart muscle cells can be seen which increases in size towards the heart. Beyond it longitudinally oriented bundles of smooth muscle cells can be noted while in the media loose circularly oriented smooth muscle cells are present.
valves of veins are located in veins of diameters from 3 to 8 mm to prevent a reflux of blood. These valves are numerous in the extremities, especially in the foot region since the orthostatic pressure (caused by the upright standing of humans) is largest here. The valves are formed by half-moon shaped circular folds of the intima including elastic and collagenous fibres and are covered by an endothelium. In case of heart insufficiency (weak heart) pressure in veins raises resulting in an incomplete closing of the valves (insufficiency of venous valves). This may lead to sac-like protrusions of the walls of veins which in most cases are unilateral and which are called varices.
Veins with special wall construction:
- in the retina, the trabecules of the spleen, and in the Pia mater small veins are seen which completely lack smooth muscle cells. This is also the case in the venous sinuses of the dura (Sinus durae matris).
- in the network of veins running next to the spermatic chord (Plexus pampiniformis), in the uterus (especially during pregnancy) and in the umbilical vein an unusual large amount of smooth muscle cells is present oriented circularly in the media and longitudinally the adventitia.
- spiral strands of smooth muscle cells of special strength which serve for (nearly) complete obliteration of the vein called arresting veins are seen on the base of the swell bodies. Under sexual arousal they serve for the swelling of these bodies causing erection in males. Further, veins with similar sphincter structures are present in the swell bodies of the nasal mucosa, in veins of the adrenal gland medulla and even in veins of the liver.

--> Venole, blood vessels, endothelial cells
--> Electron microscopic atlas Overview
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One image was kindly provided by Prof. H. Wartenberg; other images, page & copyright H. Jastrow.